1. Field of the Invention
This invention lies in the field of ceramics that are composites of silicon nitride, silicon carbide, and boron nitride, and relates in particular to methods for preparing high-density, high-strength, creep-resistant materials of these composites.
2. Description of the Prior Art
Ceramics that are composites of silicon nitride, silicon carbide, and boron nitride are used in a variety of applications, ranging from microelectromechanical devices to rocket re-entry vehicles in the aerospace industry. These composites offer high thermal stability and high resistance to oxidation, characteristics that are highly desirable for miniature high-speed high-precision devices such as nanoturbines as well as load-carrying structural surfaces that are exposed to extremely high temperatures during use.
These composites have been prepared in the prior art by the pyrolysis of hybrid organic/inorganic precursors, including mixtures of polycarbosilazanes and polymeric boron-containing carbosilanes, to achieve amorphous powders that are then densified by methods such as hot-pressing, gas-pressure sintering, and hot isostatic pressing. The microstructures of the composites produced by these methods generally consist of micron-sized or sub-micron-sized crystalline grains with inclusions of nano-sized crystals of silicon nitride dispersed through the larger grains. The term “micron-sized or sub-micron-sized” refers to grains having diameters that are greater than 100 nm, particularly 150 nm or above, while the term “nano-sized” refers to grains whose diameters are substantially less than 100 nm, particularly 50 nm or below. To increase the densification that occurs during these procedures, densification aids have been added to the amorphous powder mixture. The densification aids are metal oxides that are liquid at the sintering temperature, examples of which are magnesium oxide (MgO), alumina (Al2O3), yttria (Y2O3), lithium oxide (LiO2), and ceria (CeO2) and other rare earth oxides. Alumina, yttria, and combinations of alumina and yttria are the most common densification aids.
While densification aids help to produce composites of greater density, they also produce a glassy phase at the grain boundaries and interfaces. The glass flows at a temperature lower than the melting temperatures of the grains themselves, and this introduces creep, i.e., the tendency of adjacent grains to move relative to each other, which is detrimental to the structural stability of the material.
The present invention seeks to address these problems by providing a method for producing a highly dense silicon nitride/silicon carbide/boron nitride composite that exhibits at most an extremely low tendency toward creep.